I can tell you about the 82qt pot. It is thicker than the 62qt, has a decent hard wall; although no as hard as a blichmann or polar ware pot but seems to me sturdy enough, the bottom is imo not as thick as the wall sides, the handles are riveted, I think it weights about 22 pounds. Overall a medium thickness pot with a lot of room to brew.

I got this pot because I wanted to brew 10 and even 15 gallons batches, I even emailed John Palmer before I bought it and he said it was fine to brew. I don't own or have any other Bayou Classic pots but I know that the 62 qt one has a smaller gauge thickness and I have not even seen a converted kegg so I can more or less compare. Either way my wife got me the pot for brewing and after seeing so many great looking Rigs here I thought either get 2 Beer kegs or 2 60qt or so stock pots to start my little project.

Imo you will get a good pot for brewing but not as good as a Blichmann or a Polar ware of course. If I had the money I'll be getting the Brew magic system or a kick ass Blichmann set up but hey I have to start somewhere.

I have no idea, but I bet the gas usage is negligible. Your efficiency will depend much more on your burner arrangement than the vessel itself.

Speculation ahead:
I think the bottom skirt may help to trap a pocket of hot air that will reduce waste heat that would otherwise wash over the sides of a pot. However with proper burner spacing, sizing, and flame adjustment, a pot can be just as if not more efficient, but I guess that the skirt gives some forgiveness with poorly setup burners.